1. Field of the Invention
This invention relates to a dispersion for electrophoretic display and an electrophoretic display device making use of the dispersion for electrophoretic display, which performs display by movement of charged electrophoretic particles between electrodes.
2. Related Background Art
In recent years, with advancement of information machinery, there is an increasing need for display devices of low power consumption and small thickness, and research and development are energetically made on display devices adapted for such need. In particular, liquid crystal display devices can change optical characteristics of liquid crystal by controlling the alignment of liquid crystal molecules electrically, and they have energetically been developed and have been commertialized as display devices that can accord with the above need.
From the viewpoints of low power consumption and less tasking eyesight, hopes are entertained of reflection type display devices. As one of them, the electrophoretic display device invented by Harold D. Lees et al. is known in the art (U.S. Pat. No. 3,612,758). Besides, an electrophoretic-display device is also disclosed in Japanese Patent Application Laid-Open No. 9-185087. This electrophoretic display device is described below.
FIGS. 5A and 5B illustrate an example of the structure of such an electrophoretic display device (an example in which particles move horizontally). The electrophoretic display device of this type is constituted of a pair of substrates 13a and 13b disposed leaving a stated space between them, a dispersion medium 12 with which the space between the substrates is filled, charged electrophoretic particles 11 stand dispersed in the dispersion medium 12, a first electrode 14a and a second electrode 14b which are so disposed as to lie along one substrate 13b, partition walls 15 so disposed as to divide the space between the substrates 13a and 13b for each pixel, and an insulating layer 16.
In this device, the charged electrophoretic particles 11 are kept charged to positive polarity or negative polarity (those shown in FIGS. 5A and 5B are kept charged to positive polarity), and hence they migrate electrically to either of the electrodes 14a and 14b in accordance with the polarity of a voltage applied to the electrodes 14a and 14b. Areas in which the charged electrophoretic particles 11 and the electrodes 14a and 14b are disposed stand colored, and hence the display device can display an image as a whole.
FIGS. 6A and 6B illustrate an example of an electrophoretic display device in which particles move vertically. In FIG. 6A, the same reference numerals as those in FIG. 5A denote like constituent members. In the device shown in FIGS. 6A and 6B, the dispersion medium 12 is colored a color different from the charged electrophoretic particles 11 so that an image can be displayed. In either of the electrophoretic display devices shown in FIGS. 5A, 5B, 6A and 6B, the distance between both substrates is kept constant by the partition walls 15. As the result, these electrophoretic display devices can perform high-contrast display at reflection-type angles of view.
In such electrophoretic display devices, there is a problem that the charged electrophoretic particles may agglomerate with time of electrophoresis or the charged electrophoretic particles may adhere to electrode surfaces and inner-wall surfaces, to cause a lowering of contrast. Thus, from the viewpoint of improving display stability, their performance is desired to be more improved.
In order to improve the display stability of electrophoretic display devices, various proposals have been made.
For example, Japanese Patent Application Laid-Open No. 1-105990 discloses a proposal that, charged electrophoretic particles of 0.001 μm to 0.1 μm in diameter are used so as to maintain the dispersibility of particles stably by the aid of the Brownian movement of charged electrophoretic particles.
Japanese Patent Application Laid-Open No. 3-53224 discloses a proposal that a vibrator fitted to a cell which makes charged electrophoretic particles undergo electrophoresis is used so as to keep the particles from agglomerating or keep the particles from adhering to the electrode surfaces and inner-wall surfaces, to maintain display quality for a long time.
Japanese Patent Application Laid-Open No. 2000-227612 discloses a proposal that hollow resin particles coated with a pigment or pigment particles made hollow or porous are used as charged electrophoretic particles to make small the difference in specific gravity between the particles and the dispersion medium to improve long-term dispersibility of the particles.
Japanese Patent Publication No. 61-25139 discloses a proposal that charged electrophoretic particles or electrode substrates are coated with a fluorine resin so as to keep the particles from agglomerating or keep the particles from adhering to the electrode surfaces to obtain a practical display device.
These proposals, however, can not be said to have sufficiently solved the above problem.